In this work the thermal aging of conducting polyaniline and polypyrrole and their blends with ZnO : (PPy/ZnO (x% w/w) with x =10, 20, 30, 40),PANI/ZnO (20% w/w) and iodine has been investigated for the application of these materials as substrates in photovoltaic cells.
Today, research is focused on the organic solar cells, in which the electrical current flow is due to molecules, which play the role of donors or acceptors of electrical charge. Organic PV cells have the advantages of easy construction, low cost and they are friendly to the environment. Indium tin oxide is used as anode in organic PVs, which is characterized by high concentration of charge carriers and is used for
the injection of positive charge carriers (holes) in the organic active layer. For all the samples conductivity followed one of the models:
1. FIT (Fluctuation Induced Tunneling), in the case that we have a granular metal structure, in which conductive grains are separated by insulating barriers. These insulating barriers are narrow enough for the carriers to tunnel through small areas where the grains are closest together and the conductivity is dominated by the thermal fluctuations of the carriers in these
areas. The relationship σ=f(T) is given by
σ=σ0 exp[-T1/T0+T].
From T1 and T0 the distance s between the grains can be determined. In this work the FIT model applied to the sample of pure polypyrrole throughout the duration of thermal degradation.
2. CELT (Charging Energy Limited Tunneling), in the case that we have a granular metal structure, through which the carriers move by tunneling effect.
T0 is related directly to the ratio s/d, where s is the main separation and d is the mean diameter of the grains and can give informations about the
shrinking of the grains during aging. The relationship between σ and T is given by:
σ=σ0 exp[-(Τ0/Τ)γ]
where σ0 and T0 are indepedent factors of the temperature and 0 < γ < 0,25.
In this work the CELT model applied for the samples of pure polyaniline, PPy/ZnO (10% w/w), PPy/ZnO (20 %w/w), PANI/ZnO (20% w/w) and PPy/I throughout the duration of thermal degradation.
3. Mott (Variable range hopping model), in the case of heterogeneous structures of amorphous materials. The conductivity takes place by thermally activated electron hopping between localized states near Fermi energy. The localization is due to the randomly distributed atoms or molecules in the material. The relationship describing the change in conductivity with T is similar to that of the CELT model, with the exponent γ is roughly equal to 0,25. In this work the Mott model applied for the samples PPy/ZnO(40% w/w) and PPy/I (sublimation with iodine vapour for 24 h).
The degreasing of conductivity σ, with thermal aging time t for a substance with a granular metal structure follows the law:
σ=σαρχ exp[-(t/τ)0,5]
where τ is the time which characterizes the aging. All our composites followed this relation, ensuring a granular metal structure. It also became apparent that heating greatly reduces the electrical conductivity of our composites. The thermal degradation of PANI at room temperature (300K) gives τ = (350 ± 50) h, though at 120 0C PANI gives τ = (18 ± 4) h, indicating a much faster degradation as it is expected. The characteristic parameter τ has been calculated for all the materials used in this work.